Two French amateur archaeologists say they have found the secret burial chamber of the Pyramid of Cheops near Cairo, the largest pyramid ever built.

According to their new book, a fourth, undiscovered room lies underneath the pyramid's so-called Queen's chamber, and is likely to have been the burial chamber for Cheops, an Egyptian pharaoh who ruled from 2560 to 2532 BC.

Cheops' final resting place has never been found despite decades of investigation at the site, but the French researchers are being denied access to the pyramid to test their theory.

Gilles Dormion, an architect by training, and Jean-Yves Verd'hurt, set out to probe the mysteries of the Great Pyramid with a first trip in 1986, returning to the site in 1998.

Using a technique called microgravimetry, which measures the density of materials, they discovered what appeared to be a cavity underneath the Queen's chamber, where they also found evidence that the stone tiling had been been moved at some point.

Japanese scientists later confirmed the existence of a cavity a few metres wide, using radar technology.

The French team suggested this was a corridor leading to a further chamber, hidden deep in the belly of the pyramid, which could be the elusive sepulchral room. But they have been unable to test their theory.

"It is still a hypothesis, but everything adds up and points to the same conclusion. We need the authorisation to carry out a search," Dormion told the French newspaper Liberation.

Egyptian authorities are currently denying them access to the pyramid on the grounds that neither is a specialist, although their project has the backing of a top French academic.

Many pharaohs built their own pyramid for their mummified body to be preserved away from human view and sacrilege.

According to the French pair, none of the pyramid's three existing rooms would have been strong enough to qualify as a royal burial chamber.

The Pyramid of Cheops, greatest of the three pyramids at Giza, stands 147 metres tall and about 230 metres across. It is 2.34 million cubic metres in size, and weighs more than 4.7 million tonnes.